Do you have a sense of touch in your hair

　　Human hair includes hair and other body hair. Whether people have a sense of hair, more precisely a sense of touch, has been debated. Some people think that hair has no feeling, on the grounds that people don’t feel pain when getting a haircut. However, that doesn’t mean the base of the hair, as well as the rest of the body, is devoid of nerves and sensations. A person can also feel pain if their hair is caught or pulled by the scissors during a haircut. Even when the hair on other parts of the human body is touched by an object or blown by the wind, people will be aware of it. This shows that hair has feeling and perception function.
　　At the World Cup in Qatar, an interesting hair-related incident happened. On November 29, 2022, in the second round of the World Cup group stage, Portugal will play against Uruguay. In the 54th minute of the second half, Fernandez of the Portuguese team took a free kick. Teammate Ronaldo jumped up to head the ball, and the ball finally fell into the goal of the Uruguay team. After the goal, Ronaldo excitedly celebrated with his teammates. Six minutes later, however, FIFA ruled that the goal belonged to Fernandez because Ronaldo hadn’t touched the ball and hadn’t changed its trajectory. From the pictures captured by the stadium’s high-definition cameras, it is impossible to see that Ronaldo’s head touched the football. Perhaps, Ronaldo’s feeling is not wrong, even though his scalp did not touch the ball, but his hair touched the ball.
　　Hair includes not only the hair shaft that exposes the epidermis, but also tissues such as hair roots and follicles below the epidermis. Hair roots grow within the skin and are surrounded by follicles. Hair follicle is a sheath-like sac composed of epithelial tissue and connective tissue. It is a sac-like structure formed by the downward growth of the epidermis, and is covered with a fibrous sheath evolved from the epidermis. The hair root and the end of the follicle are called the hair bulb. The cell division of the hair bulb is active, and it is the growth point of the hair. The bottom of the hair bulb is sunken into which connective tissue protrudes, forming the dermal papilla. The dermal papilla contains capillaries and nerve endings, which can provide nutrition for the hair bulb and have sensory functions.
　　The structure and physiological function of hair determine that hair is tactile. The hair follicles are extremely sensitive to touch, which helps to increase the alertness of the head. When the fingers are playing with the hair or the wind blows the hair, people will feel that their head may touch or will touch something, and they need to take countermeasures or avoid it (when it is judged that it is a foreign body injury), or Welcome in (such as the touch of lovers and relatives). Hair on other parts of the body also has this sense of touch.
　　The sense of touch in the hair is also related to the muscles surrounding the hair follicle. On one side of the hair follicle is a bundle of obliquely running smooth muscles, the arrector pili muscles. One end of the arrector pili muscle is connected to the lower part of the hair follicle, and the other end is connected to the superficial layer of the dermis. When the arrector pili muscle contracts, the hair can be erected. This is a special sense of touch, not actually touching anything, but a nervous reaction to feeling some kind of danger or preparation to attack, and sometimes causes the hair to stand on end.
　　The literary description of this phenomenon is “angry rushing to the crown”, which first appeared in Sima Qian’s “Historical Records: Biography of Lian Po Lin Xiangru”: “Xiangru stood on the pillar because he was holding a biscuit, and his anger rushed to the crown.” Human pili muscles are not Controlled by the somatic nerves, it is controlled by the adrenergic sympathetic nerves (a type of autonomic nerves). When a person is angry, frightened or exposed to external stimuli such as cold air, the sympathetic nerve will be excited and the level of adrenaline will increase. At this time, the arrector pili muscle contracts, causing the hair to stand on end.
　　With the deepening of natural evolution, the human arrector pili muscle began to degenerate, and its contraction force can no longer make the hair stand up, but it can also make the fine hair stand up. In animals, their arrector pili muscles allow the hair to stand on end. When a dog is threatened, the hairs on its spine stand on end, ready to fight. Tigers, cats, wolves, etc. will also erect their hairs when they are ready to attack, which is also the function of the arrector pili muscle. Judging from the performance of the nerves in the hair follicle and the arrector pili muscles, the hair will be innervated by two types of nerves, the somatic nerve and the autonomic nerve.
　　Football players need to head the ball frequently, and when the player is about to head the ball, there will be two nerve sensations. One is that after the hair touches the ball, the somatic nerves in the dermal papilla can sense the movement of the ball. Another situation is that when the head has not yet touched the ball, the sympathetic nerves are tense due to the preparation for touching the ball, so the arrector pili muscles contract, making the hair “stand up” and making the head feel tactile. However, the tension and anger of modern people can seldom make hair stand on end, but at most make fine hair stand on end. Therefore, the hair touching the ball that Ronaldo felt may have really touched the football, or it may be the “touch” caused by sympathetic nervous tension.
　　Studying the relationship between hair and sensation could lead to new ways to relieve pain and treat disease. Almost everyone has this experience: when a certain part of the body hurts, such as the face, rubbing the painful part with your hands can relieve some pain; when you have a headache, pressing the temples with your hands can also relieve the headache. From a physiological point of view, when pressing and rubbing the painful part, the signal input through the sensory nerve will quiet the pain response cells in the brain, thereby relieving the pain. In the past, this explanation was just inference, and now researchers have found evidence of it in the brains of mice.
　　The researchers found that when mice feel pain in their cheeks, they rub their paws on their faces to reduce painful stimuli, which is similar to the rubbing and rubbing movements of humans when they are in pain. Past research has shown that the neural response to rubbing and touching a painful area to relieve pain may start in the spinal cord. Because when rubbed and touched, pain-responsive neurons in the spinal cord respond with a weakened signal, and the brain is also involved. However, this neural pathway is not well understood. Now, researchers have found that studying this pain-reducing neural pathway can be explored through sensory transmission through the whiskers of mice. Mice have whiskers to explore their surroundings. Their whiskers move in a rhythmic movement—back and forth or side to side—to sense the road they are traveling on and whether there is danger around them. This whisker movement is called “whisker vibration”. Inspired by this, neuroscientists at MIT’s McGovern Institute believe that whisker vibrations send information to the brain in the form of tactile signals.
　　In experiments, the researchers found that whisker vibrations changed the way mice responded to a heat source, such as a soldering iron, burning their faces. In general, mice rub their paws against their faces when exposed to a heat source to relieve pain. In experiments, when mice were stimulated with heat and simultaneously prompted to vibrate their whiskers, the mice felt less pain, or even completely ignored the painful stimulus. Looking at the somatosensory cortex of the brain that processes touch and pain signals in mice, the researchers found changes in neural signaling when the mice performed whisker vibrations, which appears to be the neural basis for pain relief. This caused the mice to respond less to painful stimuli, suggesting that whisker vibrations have a pain-relieving effect, just as they do when they rub their paws against painful areas.
　　Although this pain relief method cannot be used for human pain relief, the researchers found that the neural pathway of this beard vibration is to send touch information from a brain area called “ventral posterior thalamus” to the somatosensory cortex of the brain. In the future, new painkillers could be developed targeting this neural pathway.
　　This research can also explain why lovers like to gently touch each other’s hair, because stroking hair can relieve anxiety and pain, make people feel at ease, happy, and also a manifestation of love.